It is known to make clear and colored lens covers for warning lights in the form of cylinders, hemispheres and cones. In these contexts the precise behavior of an optical ray passing through the window is seldom significant, slight distortions are easily tolerated and indeed, some diffusion may be required or deliberately introduced.
For a device such as a long range camera, however, curvature in the window introduces progressively varying refractional effects across the field of view. This impairs the geometry of the incident beam at the camera by disturbing the notional parallelism and distribution of the incoming beam, and the resulting degradation of the image can become intolerable.